U.S. patent application number 17/603791 was filed with the patent office on 2022-07-07 for method for producing water-absorbing resin particles.
This patent application is currently assigned to Sanyo Chemical Industries, Ltd.. The applicant listed for this patent is Sanyo Chemical Industries, Ltd.. Invention is credited to Toru Miyajima, Eiji Morita, Kazumitsu Suzuki.
Application Number | 20220212165 17/603791 |
Document ID | / |
Family ID | 1000006283284 |
Filed Date | 2022-07-07 |
United States Patent
Application |
20220212165 |
Kind Code |
A1 |
Morita; Eiji ; et
al. |
July 7, 2022 |
METHOD FOR PRODUCING WATER-ABSORBING RESIN PARTICLES
Abstract
Provided is a method for efficiently producing recycled
water-absorbing resin particles from, as a raw material, discarded
water-absorbing resin particles derived from used sanitary
supplies, etc., the recycled water-absorbing resin particles having
decreased little in absorption property and having various
excellent properties. The method for producing water-absorbing
resin particles of the present invention comprises a polymerization
step (I) in which an aqueous gel containing a crosslinked polymer
(A) of a water-soluble vinyl monomer (a1) is obtained, a gel
reduction step (II) in which the aqueous gel is reduced into
particles to obtain aqueous-gel particles, a step (Va) in which
resin particles including the crosslinked polymer (A) and obtained
from the aqueous-gel particles are mixed with a
surface-crosslinking agent (d), and a reaction step (Vb) in which
the surface-crosslinking agent (d) is reacted.
Inventors: |
Morita; Eiji; (Kyoto-shi,
JP) ; Miyajima; Toru; (Kyoto-shi, JP) ;
Suzuki; Kazumitsu; (Kyoto-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sanyo Chemical Industries, Ltd. |
Kyoto-shi |
|
JP |
|
|
Assignee: |
Sanyo Chemical Industries,
Ltd.
Kyoto-shi
JP
|
Family ID: |
1000006283284 |
Appl. No.: |
17/603791 |
Filed: |
March 10, 2020 |
PCT Filed: |
March 10, 2020 |
PCT NO: |
PCT/JP2020/010234 |
371 Date: |
October 14, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 20/3021 20130101;
C08F 120/06 20130101; C08J 3/075 20130101; B01J 20/267 20130101;
B01J 20/28047 20130101; B01J 2220/4887 20130101; B01J 2220/44
20130101; C08J 11/04 20130101; B01J 20/261 20130101; B01J 20/3085
20130101; C08J 3/245 20130101 |
International
Class: |
B01J 20/26 20060101
B01J020/26; B01J 20/28 20060101 B01J020/28; B01J 20/30 20060101
B01J020/30; C08F 120/06 20060101 C08F120/06; C08J 11/04 20060101
C08J011/04; C08J 3/075 20060101 C08J003/075; C08J 3/24 20060101
C08J003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 16, 2019 |
JP |
2019-077885 |
Oct 15, 2019 |
JP |
2019-188564 |
Claims
1. A method for producing water-absorbent resin particles, the
method comprising a polymerization step (I) of polymerizing a
monomer composition comprising a water-soluble vinyl monomer (a1)
and a crosslinking agent (b) to obtain a hydrous gel comprising a
crosslinked polymer (A), a gel chopping step (II) of chopping the
hydrous gel to obtain hydrous gel particles, a step (Va) of mixing
resin particles comprising the crosslinked polymer (A) obtained
from the hydrous gel particles with a surface-crosslinking agent
(d), and a reaction step (Vb) of reacting with a
surface-crosslinking agent (d), wherein a separately prepared
decomposition product (B) of water-absorbent resin particles
comprising a crosslinked polymer (A') comprising a water-soluble
vinyl monomer (a1') and a crosslinking agent (b') as essential
constitutional units that is obtained by treating said
water-absorbent resin particles in a step of decomposing the
water-absorbent resin particles by a mechanical action and/or a
chemical action capable of breaking a chemical bond is added in at
least one of stage of the polymerization step (I), the stage of the
gel chopping step (II), and the stage of after the step (II) but
before the surface-crosslinking reaction step (Vb).
2. The method for producing water-absorbent resin particles
according to claim 1, wherein the decomposition product (B) is
added in the polymerization step (I) and/or the gel chopping step
(II).
3. The method for producing water-absorbent resin particles
according to claim 1, wherein in the gel chopping step (II), the
decomposition product (B) of water-absorbent resin particles is
added before and/or at the same time as the gel chopping.
4. The method for producing water-absorbent resin particles
according to claim 1, wherein the water-solubles content of the
decomposition product (B) of water-absorbent resin particles is 20%
by weight or more.
5. The method for producing water-absorbent resin particles
according to claim 1, wherein the decomposition product (B) of
water-absorbent resin particles is contained in an amount of 1 to
50% by weight relative to the total weight of the monomer
composition.
6. The method for producing water-absorbent resin particles
according to claim 1, wherein the decomposition product (B) of
water-absorbent resin particles is a decomposition product obtained
by decomposition-treating water-absorbent resin particles
comprising the crosslinked polymer (A') comprising the
water-soluble vinyl monomer (a1') and the crosslinking agent (b')
as essential constitutional units by at least one treatment method
selected from the group consisting of oxidation treatment,
photolysis treatment and hydrolysis treatment.
7. The method for producing water-absorbent resin particles
according to claim 6, wherein the decomposition product (B) of
water-absorbent resin particles is a decomposition product (B)
obtained by decomposition treatment in water by a method involving
oxidation treatment with a water-soluble redox agent.
8. The method for producing water-absorbent resin particles
according to claim 1, wherein the decomposition product (B) of
water-absorbent resin particles is a decomposition product obtained
from water-absorbent resin particles contained in used sanitary
goods.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing
water-absorbent resin particles. More specifically, the present
invention relates to a method for producing water-absorbent resin
particles that makes it possible in its process to recycle
water-absorbent resin particles of used sanitary goods.
BACKGROUND ART
[0002] As the used amount of sanitary goods increases, the problem
of disposal of waste sanitary goods after use is becoming serious.
Sanitary goods, especially disposable diapers, have rapidly spread
as indispensable goods in an aging society with fewer children, and
the consumption thereof has rapidly increased. Regarding waste
disposal of sanitary goods after use, paper diapers and the like
are usually incinerated, but since the proportion of moisture in
the diapers is close to about 80%, large combustion energy is
required for incineration. For this reason, a large load is applied
to the incinerator itself in the treatment, which results in a
cause of shortening the lifetime of the incinerator. In addition,
incineration treatment leads to air pollution and global warming,
and also causes a load on the environment. Therefore, improvement
is strongly desired.
[0003] In order to solve the above problems, studies have been
conducted to recover and reuse members from used sanitary goods.
Usually, sanitary goods contain absorbers composed of pulp fibers
and water-absorbent resin particles, and it is necessary to
separate the pulp fibers from the water-absorbent resin particles
for reuse as members. However, since the water-absorbent resin
particles in the absorber of the used sanitary goods absorb water
and results in a swollen gel state, it is difficult to separate the
water-absorbent resin particles as they are. For this reason, a
technique has been proposed in which water-absorbent resin
particles are decomposed and solubilized to separate pulp fibers
from solubilized components of the water-absorbent resin particles,
and for example, there is a technique in which sanitary goods
containing pulp fibers and water-absorbent resin particles are
treated with an ozone-containing aqueous solution to decompose and
solubilize the water-absorbent resin particles, and then pulp
fibers are recovered (Patent Documents 1 and 2). As a technique for
decomposing and solubilizing water-absorbent resin particles, a
technique using an oxidizing agent such as hydrogen peroxide as
means for decomposition (Patent Documents 3 to 5) is known.
PRIOR ART DOCUMENTS
Patent Documents
[0004] Patent Document 1: JP-A-2016-881
[0005] Patent Document 2: JP-A-2017-209675
[0006] Patent Document 3: JP-A-4-317784
[0007] Patent Document 4: JP-A-6-313008
[0008] Patent Document 5: JP-A-2003-321574
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0009] All of the techniques for recycling used sanitary goods
described above are intended to recover pulp fibers and utilize the
pulp fibers as recycled pulp, and the decomposed and solubilized
components of water-absorbent resin particles are discarded or are
recycled as solid fuel or the like, and it is difficult to say that
those techniques are sufficient from the viewpoint of resource
saving and reduction in environmental load. In addition, the
conventional technique of decomposing and solubilizing
water-absorbent resin particles is intended to provide a disposal
method with less environmental load, and there is room for
improvement from the viewpoint of recycling.
[0010] An object of the present invention is to provide a method
for efficiently producing recycled water-absorbent resin particles
having small deterioration in absorption characteristics and
various superior characteristics by using a waste of
water-absorbent resin particles derived from used sanitary goods or
the like as a raw material.
Solutions to the Problems
[0011] The present invention is a method for producing
water-absorbent resin particles, the method comprising a
polymerization step (I) of polymerizing a monomer composition
comprising a water-soluble vinyl monomer (a1) and a crosslinking
agent (b) to obtain a hydrous gel comprising a crosslinked polymer
(A), a gel chopping step (II) of chopping the hydrous gel to obtain
hydrous gel particles, a step (Va) of mixing resin particles
comprising the crosslinked polymer (A) obtained from the hydrous
gel particles with a surface-crosslinking agent (d), and a reaction
step (Vb) of reacting with a surface-crosslinking agent (d),
wherein a separately prepared decomposition product (B) of
water-absorbent resin particles comprising a crosslinked polymer
(A') comprising a water-soluble vinyl monomer (a1') and a
crosslinking agent (b') as essential constitutional units
(hereinafter also referred to as decomposition product (B) of
water-absorbent resin particles or decomposition product (B)) that
is obtained by treating said water-absorbent resin particles in a
step of decomposing the water-absorbent resin particles by a
mechanical action and/or a chemical action capable of breaking a
chemical bond is added in at least one of the stage of the
polymerization step (I), the stage of the gel chopping step (II),
and the stage of after the step (II) but before the
surface-crosslinking reaction step (Vb) with the
surface-crosslinking agent (d).
Advantages of the Invention
[0012] The method for producing water-absorbent resin particles of
the present invention is capable of producing water-absorbent resin
particles having less deteriorated absorption characteristics and
various superior characteristics even when using in its process a
waste of water-absorbent resin particles discharged at the time of
recycling of sanitary goods, and contributes to resource saving and
reduction in environmental load.
MODE FOR CARRYING OUT THE INVENTION
[0013] The method for producing water-absorbent resin particles of
the present invention is required to have the above-described
essential steps, and may further include other steps unless
departing from the gist of the present invention. Further, the
process of the present invention may include a transportation stage
and a storage stage unless departing from the gist.
[0014] The water-soluble vinyl monomer (a1) as used in the present
invention is not particularly limited, and there can be used such
conventional monomers as vinyl monomers having at least one
water-soluble substituent and an ethylenically unsaturated group
disclosed in paragraphs 0007 to 0023 of Japanese Patent No. 3648553
(e.g., anionic vinyl monomers, nonionic vinyl monomers, and
cationic vinyl monomers), anionic vinyl monomers, nonionic vinyl
monomers, and cationic vinyl monomers disclosed in paragraphs 0009
to 0024 of JP-A-2003-165883, and vinyl monomers having at least one
group selected from the group consisting of a carboxy group, a
sulfo group, a phosphono group, a hydroxy group, a carbamoyl group,
an amino group, and an ammonio group disclosed in paragraphs 0041
to 0051 of JP-A-2005-75982.
[0015] The water-soluble vinyl monomer (a1) is preferably an
anionic vinyl monomer, more preferably a vinyl monomer having a
carboxy (salt) group, a sulfo (salt) group, an amino group, a
carbamoyl group, an ammonio group or a mono-, di- or
tri-alkylammonio group. Among these, vinyl monomers having a
carboxy (salt) group or a carbamoyl group are more preferred,
(meth)acrylic acid (salt) and (meth)acrylamide are even more
preferred, and (meth)acrylic acid (salt) is particularly preferred,
and acrylic acid (salt) is most preferred.
[0016] The "carboxy (salt) group" means a "carboxy group" or a
"carboxylate group", and the "sulfo (salt) group" means a "sulfo
group" or a "sulfonate group." The (meth)acrylic acid (salt) means
acrylic acid, a salt of acrylic acid, methacrylic acid, or a salt
of methacrylic acid and the (meth)acrylamide means acrylamide or
methacrylamide. Examples of such salts include salts of alkali
metal (lithium, sodium, potassium, etc.), salts of alkaline earth
metal (magnesium, calcium, etc.), or ammonium (NH.sub.4) salts.
Among these salts, salts of alkali metals and ammonium salts are
preferred from the viewpoint of absorption characteristics, etc.,
salts of alkali metals are more preferred, and sodium salts are
particularly preferred.
[0017] When an acid group-containing monomer such as acrylic acid
or methacrylic acid is used as the water-soluble vinyl monomer
(a1), a part of the acid group-containing monomer may be
neutralized with a base. As a base for neutralization, alkali metal
hydroxides, such as sodium hydroxide and potassium hydroxide, and
alkali metal carbonates, such as sodium carbonate, sodium
bicarbonate, and potassium carbonate, can usually be used. In the
step of producing water-absorbent resin particles, the
neutralization may be performed either before or during
polymerization of the acid group-containing monomer, or after
polymerization, the resulting acid group-containing polymer may be
neutralized in the state of a hydrous gel containing a crosslinked
polymer (A) described later.
[0018] When an acid group-containing monomer is used, the degree of
neutralization of the acid group is preferably 50 to 80 mol %. When
the degree of neutralization is less than 50 mol %, a hydrous gel
polymer with high tackiness is to be obtained, so that the
workability in production and use may be deteriorated. Moreover,
water-absorbent resin particles with a reduced water retention
capacity may be obtained.
[0019] On the other hand, when the degree of neutralization exceeds
80%, a resin with a high pH is to be obtained and the safety to the
skin of a human body may be concerned.
[0020] In addition to the water-soluble vinyl monomer (a1), an
additional vinyl monomer (a2) copolymerizable therewith can be
contained as a constitutional unit of the crosslinked polymer (A).
The additional vinyl monomer (a2) may be used singly or two or more
of the same may be used in combination.
[0021] The additional copolymerizable vinyl monomer (a2) is not
particularly limited and conventional hydrophobic vinyl monomers
(e.g., hydrophobic vinyl monomers disclosed in paragraphs 0028 to
0029 of Japanese Patent No. 3648553, vinyl monomers disclosed in
paragraph 0025 of JP-A-2003-165883 and paragraph 0058 of
JP-A-2005-75982) can be used, and specifically, for example, the
following vinyl monomers (i) to (iii) can be used.
[0022] (i) Aromatic ethylenic monomers having 8 to 30 carbon
atoms
[0023] Styrenes, such as styrene, .alpha.-methylstyrene,
vinyltoluene and hydroxystyrene, vinylnaphthalene, and halogenated
forms of styrene, such as dichlorostyrene, etc.
[0024] (ii) Aliphatic ethylenic monomers having 2 to 20 carbon
atoms
[0025] Alkenes (e.g., ethylene, propylene, butene, isobutylene,
pentene, heptene, diisobutylene, octene, dodecene, and octadecene),
and alkadienes (e.g., butadiene and isoprene), etc.
[0026] (iii) Alicyclic ethylenic monomers having 5 to 15 carbon
atoms
[0027] Monoethylenically unsaturated monomers (e.g., pinene,
limonene, and indene); and polyethylenic vinyl monomers (e.g.,
cyclopentadiene, bicyclopentadiene, and ethylidene norbornene),
etc.
[0028] From the viewpoint of absorption performance, etc., the
content of the additional vinyl monomer (a2) unit is, based on the
number of moles of the water-soluble vinyl monomer (a1) unit,
preferably 0 to 5 mol %, more preferably 0 to 3 mol %, even more
preferably 0 to 2 mol %, and particularly preferably 0 to 1.5 mol
%, and from the viewpoint of absorption performance, etc., the
content of the additional vinyl monomer (a2) is most preferably 0
mol %.
[0029] The crosslinking agent (b) is not particularly limited, and
conventional crosslinking agents (e.g., crosslinking agents having
two or more ethylenically unsaturated groups, crosslinking agents
having at least one functional group capable of reacting with a
water-soluble substituent and having at least one ethylenically
unsaturated group, and crosslinking agents having at least two
functional groups capable of reacting with a water-soluble
substituent disclosed in paragraphs 0031 to 0034 of Japanese Patent
No. 3648553, crosslinking agents having two or more ethylenically
unsaturated groups, crosslinking agents having an ethylenically
unsaturated group and a reactive functional group, and crosslinking
agents having two or more reactive substituents disclosed in
paragraphs 0028 to 0031 of JP-A-2003-165883, crosslinkable vinyl
monomers disclosed in paragraph 0059 of JP-A-2005-75982, and
crosslinkable vinyl monomers disclosed in paragraphs 0015 to 0016
of JP-A-2005-95759) can be used. Among these, from the viewpoint of
absorption performance, etc., crosslinking agents having two or
more ethylenically unsaturated groups are preferable,
poly(meth)allyl ethers of polyhydric alcohols having 2 to 40 carbon
atoms, (meth)acrylates of polyhydric alcohols having 2 to 40 carbon
atoms, (meth)acrylamides of polyhydric alcohols having 2 to 40
carbon atoms are more preferable, polyallyl ethers of polyhydric
alcohols having 2 to 40 carbon atoms are particularly preferable,
and pentaerythritol triallyl ether is most preferable. The
crosslinking agent (b) may be used singly or two or more of the
same may be used in combination.
[0030] Water-absorbent resin particles, while facilitating breakage
of molecules by radical species or strong bases and maintaining
superior absorption performance, lead to improvement of the
efficiency of decomposition and solubilization thereof. In order to
facilitate the breakage of the molecule, it is preferable to use a
crosslinking agent (b1) having a polyether chain as an essential
component in the molecule among the crosslinking agents (b). As for
the (b1), one kind of the crosslinking agent (b1) may be used
singly, two or more kinds thereof may be used in combination, or a
crosslinking agent (b) other than the (b1) may be used in
combination.
[0031] Examples of the polyether chain include a polyoxyethylene
chain, a polyoxypropylene chain, a polyoxytetramethylene chain, and
copolymerized polyether chains thereof. Among these, a polyether
chain containing a polyoxyethylene chain is preferable from the
viewpoint of improving the efficiency of decomposition and
solubilization of the water-absorbent resin. In the case of a
polyether chain containing a polyoxyethylene chain, the content of
an oxyethylene chain in the polyether chain is preferably 50% by
weight or more, and a polyoxyethylene polyoxypropylene chain is
more preferable. Herein, the polyoxyethylene polyoxypropylene chain
is not particularly limited as long as it is a chain constituted of
an oxyethylene unit and an oxypropylene unit.
[0032] The crosslinking agent (b1) preferably has two or more
crosslinkable functional groups of one or more type selected from
the group consisting of hydroxyl group, allyl group, acryloyl
group, alkoxysilyl group, carboxyl group, epoxy group, isocyano
group, N-methylol group, and N-alkoxymethyl group per molecule.
[0033] From the viewpoint of improving the efficiency of
decomposition and solubilization of the water-absorbent resin, the
crosslinkable functional group is preferably an allyl group, an
acryloyl group, a vinyl ether group, an alkoxysilyl group, a
carboxyl group, or an epoxy group, and more preferably an allyl
group, an acryloyl group, or a vinyl ether group.
[0034] Specific examples of the crosslinking agent (b1) include
ALKOX CP-A1H and ALKOX CP-A2H (both manufactured by Meisei Chemical
Works, Ltd.), and A-1000 (polyethylene glycol diacrylate, number
average molecular weight: 1108) and A-BPE-30 (diacrylate of 30 mol
ethylene oxide adduct of bisphenol A, number average molecular
weight: 1133) (both manufactured by SHIN-NAKAMURA CHEMICAL Co.,
Ltd.). From the viewpoint of decomposability, ALKOX CP-A1H and
ALKOX CP-A2H are preferable.
[0035] The crosslinking agent (b1) has a polyether chain as an
essential component in the molecule, and the number average
molecular weight thereof is preferably 100 to 150,000, and more
preferably 200 to 100,000 from the viewpoint of improving the
efficiency of decomposition and solubilization and from the
viewpoint of water absorption performance of the water-absorbent
resin particles.
[0036] The number average molecular weight (hereinafter abbreviated
as Mn) in the present invention is measured by gel permeation
chromatography (hereinafter abbreviated as GPC). The measurement
shall follow the following measurement conditions.
<GPC Measurement Conditions>
[0037] GPC instrument: HLC-8120GPC manufactured by Tosoh
Corporation
[0038] Column: two TSKgel GMHXL+TSKgel Multipore HXL-M,
manufactured by Tosoh Corporation
[0039] Solvent: tetrahydrofuran (THF)
[0040] Detection device: refractive index detector
[0041] Reference substance: standard polystyrene (TSKstandard
POLYSTYRENE) manufactured by Tosoh Corporation
[0042] The content (mol %) of the crosslinking agent (b) units is
preferably 0.001 to 5, more preferably 0.005 to 3, and particularly
preferably 0.01 to 1 based on the number of moles of the
water-soluble vinyl monomer (a1) units or, in the case where the
additional vinyl monomer (a2) is used, the total number of moles of
(a1) and (a2). Within these ranges, the absorption performance is
further improved.
[0043] The method for producing the water-absorbent resin particles
of the present invention comprises a polymerization step (I) of
polymerizing a monomer composition comprising the water-soluble
vinyl monomer (a1) and the crosslinking agent (b) described above
to obtain a hydrous gel comprising a crosslinked polymer (A).
[0044] As the polymerization step (I), a hydrous gel containing the
crosslinked polymer (A) (a hydrous gel composed of a crosslinked
polymer containing water) can be obtained by known solution
polymerization (adiabatic polymerization, thin film polymerization,
spray polymerization, etc.; JP-A-55-133413, etc.) or known
suspension polymerization or inverse phase suspension
polymerization (JP-B-54-30710, JP-A-56-26909, JP-A-1-5808, etc.).
The crosslinked polymer (A) may be used singly or a mixture of two
or more species may be used in combination.
[0045] Of the polymerization methods, preferred is the solution
polymerization method, and the aqueous solution polymerization
method is particularly preferred because it does not need use of an
organic solvent, etc. and it is advantageous in production cost
aspect, and an adiabatic aqueous solution polymerization method is
most preferred in that water-absorbent resin particles having a
large water retention capacity and a small amount of water-soluble
components are obtained and the temperature control during
polymerization is unnecessary.
[0046] When performing aqueous solution polymerization, a mixed
solvent comprising water and an organic solvent can be used, and
examples of the organic solvent include methanol, ethanol, acetone,
methyl ethyl ketone, N,N-dimethylformamide, dimethyl sulfoxide, and
mixtures of two or more thereof. When performing aqueous solution
polymerization, the usage (% by weight) of an organic solvent is
preferably 40 or less, and more preferably 30 or less, based on the
weight of water.
[0047] When using an initiator for polymerization, a conventional
initiator for radical polymerization can be used and examples
thereof include azo compounds [e.g., azobisisobutyronitrile,
azobiscyanovaleric acid, and 2,2'-azobis(2-amidinopropane)
hydrochloride], inorganic peroxides (e.g., hydrogen peroxide,
ammonium persulfate, potassium persulfate, and sodium persulfate),
organic peroxides [e.g., benzoyl peroxide, di-tert-butyl peroxide,
cumene hydroperoxide, succinic acid peroxide, and di(2-ethoxyethyl)
peroxydicarbonate], and redox catalysts (combinations of a redox
agent such as alkali metal sulfite or bisulfite, ammonium sulfite,
ammonium bisulfite and ascorbic acid, and an oxidizing agent such
as alkali metal persulfates, ammonium persulfate, hydrogen
peroxide, and organic peroxides. These catalysts may be used singly
and two or more species thereof may be used in combination.
[0048] The used amount (% by weight) of the polymerization
initiator is preferably 0.0005 to 5, and more preferably 0.001 to
2, based on the weight of the water-soluble vinyl monomer (a1) or,
in the case of using an additional vinyl monomer (a2) as well, the
total weight thereof.
[0049] In polymerization, a polymerization controlling agent
represented by a chain transfer agent may be used together as
necessary, and specific examples thereof include sodium
hypophosphite, sodium phosphite, alkylmercaptans, alkyl halides,
and thiocarbonyl compounds. Such polymerization controlling agents
may be used singly and two or more species thereof may be used in
combination.
[0050] The amount (% by weight) of the polymerization controlling
agent used is preferably 0.0005 to 5, and more preferably 0.001 to
2, based on the weight of the water-soluble vinyl monomer (a1) or,
in the case of using an additional vinyl monomer (a2) as well, the
total weight thereof.
[0051] When applying a suspension polymerization method or an
inverse phase suspension polymerization method as a polymerization
method, the polymerization may be carried out in the presence of a
dispersing agent or a surfactant as necessary. In the case of an
inverse phase suspension polymerization method, the polymerization
can be carried out using a hydrocarbon solvent such as xylene,
n-hexane, and n-heptane.
[0052] The polymerization onset temperature can appropriately be
adjusted depending on the type of the catalyst to be used, and it
is preferably 0 to 100.degree. C., and more preferably 2 to
80.degree. C.
[0053] The gel chopping step (II) is a step of chopping the hydrous
gel comprising the crosslinked polymer (A) obtained in the
polymerization step to obtain hydrous gel particles. The size
(longest diameter) of the hydrous gel particles after the gel
chopping step is preferably 50 .mu.m to 10 cm, more preferably 100
.mu.m to 2 cm, and particularly preferably 500 .mu.m to 1 cm.
Within these ranges, dryability during a drying step is further
improved.
[0054] Gel chopping can be performed by a known method, and
chopping can be performed using a pulverizing apparatus (e.g.,
kneaders, universal mixers, single screw or twin screw kneading
extruders, mincing machines and meat choppers).
[0055] As described above, the hydrous gel of the acid
group-containing polymer obtained after polymerization may also be
neutralized by mixing a base during the gel chopping step. The base
to be used when neutralizing the acid group-containing polymer and
a preferred range of the degree of neutralization are the same as
those in the case of using an acid group-containing monomer.
[0056] The hydrous gel particles obtained in the gel chopping step
(II) are preferably obtained as resin particles containing the
crosslinked polymer (A) via a drying step (III), a pulverization
classification step (IV), or the like before the later-described
surface-crosslinking step (Va and Vb described later). Furthermore,
other steps may be further included unless departing from the gist
of the production method of the present invention. Hereinafter, the
production method of the present invention will be described mainly
in order with passage of time.
[0057] As a method to be applied to the drying step (III) of drying
the hydrous gel particles (including distillation of solvent), a
method of drying them with hot blast having a temperature of 80 to
300.degree. C., a thin film drying method using a drum dryer heated
to 100 to 230.degree. C. or the like, a (heating) reduced pressure
drying method, a freeze-drying method, a drying method using
infrared radiation, decantation, filtration, etc. can be applied,
and two or more of them may be used in combination. The heat
sources (steam, heat medium, etc.) of these dryers are not
particularly limited. The drying method is not particularly limited
and may be either a batch drying method or a continuous drying
method.
[0058] When water is contained in the solvent, the water content (%
by weight) after drying is preferably 0 to 20, and more preferably
1 to 10 based on the weight of the crosslinked polymer (A). Within
these ranges, the absorption performance is further improved.
[0059] The water content can be determined from the weight loss of
a sample before and after heating when heating with an infrared
moisture content analyzer (JE400 manufactured by KETT, or the like;
120.+-.5.degree. C., 30 minutes, atmosphere humidity before
heating: 50.+-.10% RH, lamp specification: 100 V, 40 W).
[0060] The method of the pulverization in the pulverization
classification step (IV) is not particularly limited and
pulverizing apparatuses (e.g., hammer type pulverizer, impact type
pulverizer, roll type pulverizer, and jet stream type pulverizer)
can be used.
[0061] The classification in the pulverization classification step
(IV) is performed to control the weight average particle diameter
and the particle size distribution of the pulverized resin
particles. The classifying apparatus is not particularly limited,
and a known means such as a vibration sieve, an in-plane motion
sieve, a movable mesh sieve, a forced agitation sieve, or a sonic
sieve is used, and a vibration sieve or an in-plane motion sieve is
preferably used. The classified resin particles may optionally
contain some other components such as a residual solvent and a
residual crosslinking component.
[0062] The production method of the present invention includes a
surface-crosslinking step (V) of surface-crosslinking resin
particles comprising the crosslinked polymer (A) with a
surface-crosslinking agent. The surface-crosslinking step (V)
includes, as essential steps, a step (Va) of mixing resin particles
with a surface-crosslinking agent (d), and a reaction step (Vb) of
reacting with a surface-crosslinking agent (d).
[0063] Therefore, in the production method of the present
invention, the stage of after the step (II) and before the
surface-crosslinking reaction step (Vb) of reacting with the
surface-crosslinking agent (d) may be a drying step (III), a
pulverization classification step (IV), a step (Va) of mixing resin
particles with a surface-crosslinking agent (d), etc.
[0064] As the surface-crosslinking agent (d), there can be used
conventional surface-crosslinking agents (e.g., polyglycidyl
compounds, polyamines, polyaziridine compounds, polyisocyanate
compounds, etc. disclosed in JP-A-59-189103; polyhydric alcohols
disclosed in JP-A-58-180233 and JP-A-61-16903; silane coupling
agents disclosed in JP-A-61-211305 and JP-A-61-252212; alkylene
carbonates disclosed in JP-A-5-508425; polyoxazoline compounds
disclosed in JP-A-11-240959; and multivalent metal salts disclosed
in JP-A-51-136588 and JP-A-61-257235). Among these
surface-crosslinking agents, polyglycidyl compounds, polyhydric
alcohols and polyamines are preferred, polyglycidyl compounds and
polyhydric alcohols are more preferred, polyglycidyl compounds are
particularly preferred, and ethylene glycol diglycidyl ether is
most preferred from the viewpoint of economic efficiency and
absorption characteristics. The surface-crosslinking agent may be
used singly or two or more species thereof may be used in
combination.
[0065] The amount (parts by weight) of the surface-crosslinking
agent used is not particularly limited because it can be varied
depending upon the type of the surface-crosslinking agent, the
conditions for crosslinking, target performance, etc.; from the
viewpoint of absorption characteristics, etc., it is preferably
0.001 to 3, more preferably 0.005 to 2, and particularly preferably
0.01 to 1.5, relative to 100 parts by weight of the resin particles
comprising the crosslinked polymer (A).
[0066] The surface-crosslinking of the crosslinked polymer (A) has
a step (Va) of mixing resin particles comprising the crosslinked
polymer (A) with the surface-crosslinking agent (d) and can be
performed by heating as necessary. Examples of the method for the
mixing include a method of uniformly mixing by use of a mixing
apparatus, such as a cylindrical mixer, a screw type mixer, a screw
type extruder, a Turbulizer, a Nauter mixer, a double-arm kneader,
a fluidization mixer, a V-type mixer, a mincing mixer, a ribbon
mixer, an air mixer, a rotating disc mixer, a conical blender, and
a roll mixer. Under the present circumstances, the
surface-crosslinking agent (d) may be used with dilution with water
and/or an arbitrary solvent.
[0067] The resin particles comprising the crosslinked polymer (A)
are mixed with the surface-crosslinking agent (d), and then the
vicinity of the surface of the resin particles is reacted with the
(d) in the reaction step (Vb). The water-absorbent resin particles
obtained by the production method of the present invention have a
structure in which the surface of a resin particle comprising the
crosslinked polymer (A) is surface-crosslinked with the
surface-crosslinking agent (d). By having the structure of
surface-crosslinked with the surface-crosslinking agent, gel
blocking can be suppressed and necessary absorption characteristics
(balance between water retention capacity and amount of absorption
under load) can be controlled. In addition, by the present reaction
step, the crosslinking density of the decomposition product (B)
increases and sufficient strength can be given to the resin
particles. The temperature of the resin particles in the reaction
step is preferably equal to or higher than room temperature from
the viewpoint of reaction rate. In addition, the powder temperature
of the (A) may have reached room temperature or higher by a
previous step, but is preferably 100.degree. C. or lower.
[0068] The heating temperature is preferably 100 to 180.degree. C.
Heating at 180.degree. C. or lower is advantageous in facility
aspect because indirect heating using steam can be employed
therefor, whereas at a heating temperature of lower than
100.degree. C., absorption performance may be deteriorated. The
heating time can appropriately be set depending on the heating
temperature, and from the viewpoint of absorption performance, it
is preferably 5 to 60 minutes. The water-absorbent resin particles
obtained by surface-crosslinking may be further surface-crosslinked
using a surface-crosslinking agent of the same type as or a
different type from the surface-crosslinking agent used first.
[0069] After crosslinking the surface of the resin particles
comprising the crosslinked polymer (A) with the
surface-crosslinking agent (d), the particle size is adjusted by
screening, if necessary.
[0070] The water-absorbent resin particles of the present invention
preferably comprise a multivalent metal salt (e). Inclusion of the
multivalent metal salt (e) can dramatically improve the absorption
performance of the water-absorbent resin particles comprising the
decomposition product (B). It is presumed that when the multivalent
metal salt (e) has a plurality of multivalent metal atoms, the
multivalent metal salt (e) interacts with the water-absorbent resin
particles comprising the decomposition product (B) at multiple
points to improve absorption performance.
[0071] Examples of the multivalent metal salt (e) include inorganic
acid salts of zirconium, aluminum or titanium, and examples of the
inorganic acid that forms the multivalent metal salt (e) include
sulfuric acid, hydrochloric acid, nitric acid, hydrobromic acid,
hydroiodic acid, and phosphoric acid. Examples of the inorganic
acid salt of zirconium include zirconium sulfate and zirconium
chloride, examples of the inorganic acid salt of aluminum include
aluminum sulfate, aluminum chloride, aluminum nitrate, ammonium
aluminum sulfate, potassium aluminum sulfate and sodium aluminum
sulfate, and examples of the inorganic acid salt of titanium
include titanium sulfate, titanium chloride and titanium
nitrate.
[0072] Among these, inorganic acid salts of aluminum are preferable
from the viewpoint of improving absorption performance under
pressure, and aluminum sulfate, aluminum chloride, potassium
aluminum sulfate and sodium aluminum sulfate are more
preferable.
[0073] From the viewpoint of absorption performance under pressure,
the used amount (% by weight) of the multivalent metal salt (e) is
preferably 0.05 to 5.0, more preferably 0.2 to 2.0, and
particularly preferably 0.35 to 1.5, based on the weight of the
crosslinked polymer (A). Herein, when the multivalent metal salt
(e) is a hydrate, the mass excluding hydration water is used as the
weight thereof.
[0074] As for the timing of adding the multivalent metal salt (e),
the multivalent metal salt (e) may be added in any step such as the
polymerization step (I), the gel chopping step (II), or the
surface-crosslinking step (V), but from the viewpoint of improving
the absorption performance under pressure, the multivalent metal
salt (e) is preferably located on the surface of the resin
particles and is preferably added in the surface-crosslinking step
(V). When the surface treatment is performed with the
surface-crosslinking agent described above, the multivalent metal
salt (e) may be added in any timing of the mixing step (Va) before
the reaction step of performing the surface treatment by
surface-crosslinking, after the reaction step (Vb), or at the same
time as the reaction step (Vb). Further, as for the facility and
the temperature for mixing the multivalent metal salt (e), the
mixing may be performed in the same manner as the
surface-crosslinking step described above.
[0075] The water-absorbent resin particles may comprise a liquid
permeation enhancer, if necessary. The liquid permeation enhancer
means a material that treats the surface of water-absorbent resin
particles by non-covalent bond interaction (ionic bond, hydrogen
bond, hydrophobic interaction, etc.), and is distinguished from the
above-described surface-crosslinking agent. The liquid permeation
enhancer is contained in the surface of the water-absorbent resin
particles and has an action of preventing inter-particle blocking
during gel swelling and improving liquid permeability.
[0076] Examples of the liquid permeation enhancer include cationic
organic polymers, inorganic particles and the multivalent metal
salt (e) described above. These may be used singly or in
combination.
[0077] The cationic organic polymer is not particularly limited,
and known cationic organic polymers disclosed as examples in WO
2017-57709 can be used.
[0078] Examples of the inorganic particles include hydrophilic
inorganic particles and hydrophobic inorganic particles. Examples
of the hydrophilic inorganic particles include particles of glass,
silica gel, silica (colloidal silica, fumed silica, etc.), clay,
etc. Examples of the hydrophobic inorganic particles include
particles of carbon fiber, kaolin, talc, mica, bentonite, sericite,
asbestos, and shirasu. Among these, hydrophilic inorganic particles
are preferable, and silica (colloidal silica, fumed silica, etc.)
is most preferable.
[0079] From the viewpoint of absorption performance, the used
amount (parts by weight) of the liquid permeation enhancer is
preferably 0.05 to 5, and more preferably 0.2 to 2.0, based on 100
parts by weight of the water-absorbent resin particles.
[0080] The water-absorbent resin particles may contain other
additives {e.g., known antiseptics, fungicides, antibacterial
agents, antioxidants, ultraviolet absorbers, colorants, fragrances,
deodorants, organic fibrous materials, etc. (disclosed in
JP-A-2003-225565, JP-A-2006-131767, etc.)}. When such an additive
is contained, the content (% by weight) of the additive is
preferably 0.001 to 10, more preferably 0.01 to 5, particularly
preferably 0.05 to 1, and most preferably 0.1 to 0.5, based on the
weight of the crosslinked polymer (A).
[0081] In the method for producing water-absorbent resin particles
of the present invention, a separately prepared decomposition
product (B) is added in at least one of the stage of the
polymerization step (I), the stage of the gel chopping step (II),
and the stage of after the step (II) but before the
surface-crosslinking reaction step (Vb) with the
surface-crosslinking agent (d), in other words, in any step before
the surface-crosslinking reaction (Vb), whereby absorption
performance deteriorated by decomposition can be recovered by the
subsequent surface-crosslinking reaction. The decomposition product
(B) is a product obtained by treating water-absorbent resin
particles in a step of decomposing the water-absorbent resin
particles by a mechanical action and/or a chemical action capable
of breaking a chemical bond.
[0082] Water-absorbent resin particles comprising a crosslinked
polymer (A') comprising a water-soluble vinyl monomer (a1') and a
crosslinking agent (b') as essential constitutional units can be
produced by a method similar to the method for producing the
water-absorbent resin particles comprising the crosslinked polymer
(A) by polymerizing the monomer composition comprising the
water-soluble vinyl monomer (a1) and the crosslinking agent (b)
described above. Here, the water-soluble vinyl monomer (a1') and
the crosslinking agent (b') may be either the same as or different
from (a1) and (b), respectively, but are preferably the same from
the viewpoint of the stability of absorption characteristics.
[0083] The decomposition product (B) of water-absorbent resin
particles is a polymer in which covalent bonds of a crosslinked
polymer (A') comprising a water-soluble vinyl monomer (a1') and a
crosslinking agent (b') as essential constitutional units are
broken by a mechanical action such as shearing and/or a chemical
action such as hydrolysis and intramolecular and/or intermolecular
chemical bonds are broken. The form of the decomposition product
(B) is not particularly limited, and examples thereof include a
powdery form, a granular form, a gel form, and an aqueous solution
form.
[0084] The number average molecular weight of the water solubles of
the decomposition product (B) of the water-absorbent resin
particles is preferably 500,000 or less from the viewpoint of
suppressing deterioration of absorption characteristics of the
water-absorbent resin particles. The molecular weight is measured,
for example, by using a gel permeation chromatography (1200 Series,
manufactured by Agilent Technologies, Ltd.) equipped with a multi
angle light scattering detector (DAWN HELEOS II, manufactured by
Shoko Scientific Co., Ltd.) (hereinafter abbreviated as GPC-MALS),
using an aqueous solution containing 0.5 M acetic acid and 0.2 M
sodium nitrate as a solvent, adjusting the sample concentration to
0.2% by weight, using a polymer-based filler (OHpak SB-806M HQ
produced by Shoko Scientific Co., Ltd.) as a column stationary
phase, and adjusting the column temperature to 40.degree. C.
[0085] The content (% by weight) of water solubles of the
decomposition product (B) of the water-absorbent resin particles is
preferably 20 or more, more preferably 25 or more, and particularly
preferably 30 or more from the viewpoint of recycle efficiency of
the decomposition product and suppression of deterioration of
absorption characteristics of the water-absorbent resin particles.
As for the upper limit of the solubles content, the upper limit of
the content of solubles in water at 25.degree. C. is preferably 90%
from the viewpoint of actual treatment. Adjusting to these ranges
leads to energy reduction and process time reduction in the
decomposition treatment step. Although the decomposition product
(B) is composed of a mixture of a water-soluble component and a
water-insoluble component, it is also possible to remove the
insoluble component and use only the soluble component, but it is
preferable to use the decomposition product for recycling without
removing one of the components from the viewpoint of recycling the
decomposition product. The water soluble content means the weight
ratio of the water-soluble component to the whole solid component
contained in the decomposition product (B) of the water-absorbent
resin particles, and the content of water solubles is measured by
the following method.
<Content of Water Solubles>
[0086] One hundred grams of physiological saline (salt
concentration: 0.9% by weight) is weighed into a 300 ml plastic
container and 1.2 g of water-absorbent resin particles are added to
the physiological saline. The container is sealed with plastic
wrapping film and then the contents are stirred with a stirrer at a
rotation speed of 500 rpm for 3 hours to prepare a water soluble
extract in which the water solubles of the water-absorbent resin
particles have been extracted. Then, the water solubles extract is
filtered using a filter paper produced by ADVANTEC Toyo Kaisha Ltd.
(product name: JIS P 3801, No. 2; thickness: 0.26 mm, retaining
particle size: 5 .mu.m). Then, 10 g of the obtained filtrate is
weighed and 40 g of ion-exchanged water is added to prepare a
sample solution. Hereinafter, a method for measuring the content of
water solubles of the water-absorbent resin particles will be
described.
[0087] Using 50 g of physiological saline (salt concentration: 0.9%
by weight) as a blank test solution, titration with an N/50 aqueous
KOH solution is carried out until the pH of the saline becomes 10.
Then, a titrated amount of the N/50 aqueous KOH solution required
until the pH of the saline solution becomes 10 ([W.sub.KOH,b] ml)
is determined. Then, titration of an N/10 aqueous HCl solution is
carried out until the pH of the saline solution becomes 2.7. And
then, a titrated amount of the N/10 aqueous HCl solution
([W.sub.HCl,b] ml) required until the pH of the saline solution
becomes 2.7 is determined.
[0088] Next, a concrete description is given to methods for
determining a titrated amount of an N/50 aqueous KOH solution
required until the pH of a sample solution becomes 10
([W.sub.KOH,S] ml) and a titrated amount of an N/10 aqueous HCl
solution required until the pH of a sample solution becomes 2.7
([W.sub.HCl,S] ml) for the sample solution described above.
[0089] For example, in the case of water-absorbent resin particles
composed of acrylic acid and a sodium salt thereof, the amount of
substance of unneutralized acrylic acid n.sub.COOH is given by:
n.sub.COOH (mol)=(W.sub.KOH,S-W.sub.KOH,b).times.(
1/50)/1000.times.5.
[0090] In addition, the total amount of substance of acrylic acid
n.sub.tot is given by:
n.sub.tot (mol)=(W.sub.HCl,S-W.sub.HCl,b).times.(
1/10)/1000.times.5.
[0091] The amount of substance of neutralized acrylic acid
n.sub.COONa is given by:
n.sub.COONa (mol)=n.sub.tot-n.sub.COOH.
[0092] Furthermore, the weight of unneutralized acrylic acid
n.sub.COOH is given by:
m.sub.COOH (g)=n.sub.COOH.times.72.
[0093] Further, the amount of substance of neutralized acrylic acid
m.sub.COONa is given by:
m.sub.COONa (g)=n.sub.COONa.times.94.
[0094] Based on the above values and the water content of the
water-absorbent resin particles used as a sample ([W.sub.H2O] % by
weight), a content of water solubles of the water-absorbent resin
particles can be calculated by the following calculation
formula.
Content of water solubles (% by
weight)={(m.sub.COOH+m.sub.COONa).times.100}/{1.2.times.(100-W.sub.H2O)}
[0095] Regarding the measurement of the content of water solubles,
there is a concern that the measurement is affected by an inorganic
acid and/or an organic acid having a carboxylic acid group, a
sulfonic acid group, or the like that coexist(s) with the water
solubles derived from the water-absorbent resin particles.
Therefore, it is necessary to quantify acid components other than
the components derived from the water-absorbent resin particles by,
for example, a colloid titration method, an ion chromatography
method, an ICP emission spectrophotometer, or a fluorescent X-ray
measurement method, which are known methods, singly or in
combination. More specifically, by producing a calibration curve in
advance for each contained acid component, the amount of the acid
component contained in the water solubles can be estimated, and the
acid component contained can be identified by making a reference to
an existing library. The content of water solubles derived from the
water-absorbent resin particles can be determined by subtracting
the content of the acid component quantified by the method
described above from the value measured for the content of water
solubles.
[0096] Examples of the method for obtaining the decomposition
product (B) of water-absorbent resin particles include a method in
which water-absorbent resin particles comprising a crosslinked
polymer (A') comprising a water-soluble vinyl monomer (a1') and a
crosslinking agent (b') as essential constitutional units are
subjected to decomposition treatment by at least one method
selected from the group consisting of oxidation treatment,
photolysis treatment and hydrolysis treatment.
[0097] The method of oxidation treatment is not particularly
limited as long as water-absorbent resin particles are decomposed
by an oxidation action, and examples thereof include ozone
treatment described in JP-A-2014-217835, hypochlorous acid (salt)
treatment described in JP-A-2013-150977, and oxidation treatment
with hydrogen peroxide, persulfuric acid, chlorine or a
water-soluble redox agent.
[0098] The "hypochlorous acid (salt)" means "hypochlorous acid" or
"hypochlorite". Examples of the hypochlorite include sodium
hypochlorite (NaClO), calcium hypochlorite (Ca(ClO).sub.2) and
potassium hypochlorite (KClO).
[0099] Examples of the water-soluble redox agent include ascorbic
acid, ascorbic acid derivatives, glutathione, catechin and
catechinic acid derivatives. Among these, ascorbic acid and
ascorbic acid derivatives are preferable from the viewpoint of
availability and improvement in solubilization rate. In the present
invention, the "water-soluble redox agent" refers to a redox agent
having a solubility in water at 20.degree. C. of 0.1 g/ml or more,
preferably 0.3 g/ml or more.
[0100] The ascorbic acid and the ascorbic acid derivatives are not
particularly limited as long as they are known, but the ascorbic
acid may be any of L-form, D-form, and DL-form, and L-ascorbic acid
is preferable from the viewpoint of availability. In addition, the
"ascorbic acid derivative" means a derivative obtained by
chemically modifying or substituting ascorbic acid and a part
thereof. Specific examples of the ascorbic acid derivatives include
ascorbic acid phosphate, ascorbic acid sulfate, and ascorbic acid
glycoside. Metal (sodium, potassium, magnesium, calcium, etc.)
salts and organic (ammonium, amine, etc.) salts of these ascorbic
acid and ascorbic acid derivatives can also be used. These ascorbic
acid and ascorbic acid derivatives may be used singly or two or
more of them may be used in combination.
[0101] The catechin and the catechin acid derivatives are not
particularly limited as long as they are known, but the catechin
may be any of (+)-catechin, (-)-catechin, a racemate, and a
catechin hydrate, and (+)-catechin is preferable from the viewpoint
of availability. In addition, the "catechin derivative" means a
derivative obtained by chemically modifying or substituting
catechin and a part thereof. Specific examples of the catechin
derivative include gallocatechin, epicatechin and epigallocatechin.
Metal (sodium, potassium, magnesium, calcium, etc.) salts and
organic (ammonium, amine, etc.) salts of these catechin and
catechin derivatives can also be used. These catechin and catechin
derivatives may be used singly or two or more of them may be used
in combination.
[0102] In a method in which decomposition treatment is performed in
water by oxidation treatment with a treatment liquid comprising a
water-soluble redox agent, the content (% by weight) of the
water-soluble redox agent in the treatment liquid is preferably
0.05 to 40, more preferably 1 to 35, and particularly preferably 1
to 30, based on the weight of the water-absorbent resin particles,
from the viewpoint of improvement in the solubilization rate of the
water-absorbent resin particles. When the content of the
water-soluble redox agent is less than 0.05, the solubilization
rate may decrease, and when the content is more than 40, it is not
economical.
[0103] The method for making the water-soluble redox agent to be
contained is not limited as long as the water-soluble redox agent
is contained at the time of decomposition treatment, and examples
thereof include a method in which the water-soluble redox agent is
contained in the water-absorbent resin particles in advance, and a
method in which the water-soluble redox agent is added before
and/or during the decomposition treatment. As for the form when
added, it may be added in either a solid or an aqueous solution,
and it is preferably added as an aqueous solution from the
viewpoint of improving the solubilization rate.
[0104] In the method in which decomposition treatment is performed
in water by oxidation treatment with a treatment liquid comprising
a water-soluble redox agent, the treatment liquid preferably
comprises iron ion and/or copper ion in addition to the
water-soluble redox agent. By containing iron ions and/or copper
ions, the solubilization rate can be improved. Although a detailed
solubilization mechanism is not clear, a mechanism is generally
known in which a redox agent such as ascorbic acid generates
hydrogen peroxide in the presence of oxygen and iron ions, and the
hydrogen peroxide further gives hydroxyl radicals by a Fenton
reaction with iron ions. Therefore, also as for the decomposition
treatment using the water-soluble redox agent, it is presumed that
radical species generated by a similar mechanism decompose and
solubilize the crosslinked polymer of the water-absorbent resin
particles.
[0105] In the decomposition treatment step, from the viewpoint of
improvement in the solubilization rate of the water-absorbent resin
particles, the content (ppm, w/w) of iron ions and/or copper ions
in the treatment liquid is preferably 0.01 to 10000 ppm, more
preferably 0.05 to 8000 ppm, and particularly preferably 0.1 to
5000 ppm, based on the weight of the water-absorbent resin
particles. When the content of the water-soluble redox agent is
less than 0.01 ppm, the solubilization rate decreases, and the
efficiency of separation and recovery of pulp fibers and the like
may decrease, and when the content is more than 10,000 ppm, it is
not economical.
[0106] Iron ions and/or copper ions may be impurities contained in
the water-absorbent resin particles themselves or other raw
materials as long as the content thereof is within the above range.
When a raw material for supplying iron ions and/or copper ions is
used, such a raw material is not particularly limited as long as it
is a water-soluble compound and it dissolves in water to generate
iron ions and/or copper ions.
[0107] The contents of the iron ions and the copper ions contained
in the water-absorbent resin particles themselves or other raw
materials can be measured, for example, by heating the raw
materials with a microwave sample decomposer in the presence of a
strong acid such as hydrochloric acid or nitric acid to dissolve
the raw materials, and then measuring the contents with an ICP
emission spectrometer (ICP-OES).
[0108] Examples of the compound that gives an iron ion include iron
chloride(II), iron chloride(III), iron lactate(II), iron
sulfate(II), iron sulfate(III), iron phosphate(II), iron
phosphate(III), iron iodide(II), iron iodate, potassium
ferricyanide, sodium ferricyanide, potassium ferrocyanide, sodium
ferrocyanide, ammonium iron citrate, iron cyanide(II), iron
oxalate, iron bromide, iron nitrate(II), iron nitrate(III), iron
hydroxide(II), and iron hydroxide(III). These compounds may be
nonhydrates or hydrates such as monohydrate, dihydrate, trihydrate,
tetrahydrate, pentahydrate, hexahydrate, heptahydrate, octahydrate,
and nonahydrate.
[0109] Examples of the compound that gives a copper ion include
copper acetate(I), copper acetate(II), copper chloride(II), copper
chloride(III), copper sulfate(I), and copper sulfate(II). These
compounds may be nonhydrates or hydrates such as monohydrate,
dihydrate, trihydrate, tetrahydrate, pentahydrate, hexahydrate,
heptahydrate, octahydrate, and nonahydrate.
[0110] In the method of decomposition treating the water-absorbent
resin particles in water by oxidation treatment with a
water-soluble redox agent, the decomposition product (B) of
water-absorbent resin particles is preferably decomposition treated
in water with a treatment liquid comprising the water-soluble redox
agent and iron ions and/or copper ions from the viewpoint of
improvement in solubilization rate. The method for adding water is
not particularly limited as long as the water-absorbent resin
particles are present in water during the decomposition treatment,
and for example, the water-absorbent resin particles may be swollen
with water in advance before the decomposition treatment, or water
may be added during the decomposition treatment.
[0111] An apparatus for the decomposition treatment is not
particularly limited as long as it is an apparatus capable of
mixing the water-absorbent resin particles with the water-soluble
redox agent, and for example, the decomposition treatment may be
performed in a stirrable treatment tank.
[0112] The temperature at which the decomposition treatment is
performed is not particularly limited as long as it is a
temperature at which the water-absorbent resin particles are
solubilized by a water-soluble redox agent, and is 20 to
100.degree. C., and by increasing the temperature, the time for
solubilization can be shortened.
[0113] Examples of the method of photolysis treatment include a
method of applying ultraviolet rays and a method of applying
ultraviolet rays in combination with a photocatalyst such as
titanium oxide.
[0114] Examples of the method of hydrolysis treatment include a
method of decomposing by the action of water in the presence of an
acid or alkali. This method is effective when a hydrolyzable
functional group such as an ester bond or an amide bond is
introduced into a main chain or a crosslinking site of a
crosslinked polymer.
[0115] The amount (% by weight) of the acid or alkali added to the
hydrolysis treatment is preferably 0.001 to 10, more preferably
0.01 to 5, and particularly preferably 0.01 to 3, based on the
weight of the crosslinked polymer (A). After the hydrolysis
treatment, neutralization treatment may be performed as
necessary.
[0116] As for the method for obtaining the decomposition product
(B) of water-absorbent resin particles, from the viewpoint of
simplicity of the decomposition treatment, oxidation treatment is
preferred, and a method of oxidation treatment with hypochlorous
acid (salt) or a method of oxidation treatment with a water-soluble
redox agent are more preferred.
[0117] From the viewpoint of resource saving and reduction in
environmental load, the decomposition product (B) of
water-absorbent resin particles is preferably a decomposition
product obtained from water-absorbent resin particles contained in
used sanitary goods. Water-absorbent resin particles contained in
used sanitary goods will be disposed, and it can be said that the
present invention has significance in making such water-absorbent
resin particles recyclable.
[0118] The sanitary goods are not particularly limited as long as
they are sanitary goods comprising water-absorbent resin particles,
but generally, the sanitary goods include an absorber composed of
pulp fibers and water-absorbent resin particles therein. Specific
examples of the sanitary goods include disposable diapers
(disposable diaper for children, disposable diaper for adults,
etc.), napkins (sanitary napkin, etc.), paper towel, pads
(incontinence pad, surgical underpad, etc.), and pet sheets (pet
urine absorbing sheet). The sanitary goods are not limited to used
products, and final products, intermediate products, etc. also can
be used.
[0119] Examples of the pulp fibers include pulp fibers
conventionally used in sanitary goods such as various fluff pulps
and cotton-like pulps. The pulp fibers are not particularly limited
with respect to their source material (needle-leaf trees, broadleaf
trees, etc.), production method (chemical pulp, semi-chemical pulp,
chemi-thermomechanical pulp, etc.), bleaching method, and form
(sheet-form tissue, etc.) are not particularly limited.
[0120] When the decomposition product of water-absorbent resin
particles is obtained from used sanitary goods, the decomposition
product of the water-absorbent resin particles may be recovered
from the sanitary goods directly or after pulverizing the sanitary
goods in advance and subjecting to decomposition treatment, or the
water-absorbent resin particles may be separated and recovered from
the sanitary goods and then subjected to a decomposition
treatment.
[0121] In the case of containing excrements such as urine and feces
in the used paper diapers or the like, it is preferable to subject
the water-absorbent resin particles to sterilization treatment from
the viewpoint of hygiene. The water-absorbent resin particles are
sterilized in the decomposition treatment step described above, but
when this sterilization is not sufficiently performed, a step of
subjecting the water-absorbent resin particles to sterilization
treatment may be provided. The sterilization treatment step may be
performed before the decomposition treatment step, or may be
performed simultaneously with the decomposition treatment step.
Specific examples of the step of subjecting the water-absorbent
resin particles to sterilization treatment include a
high-temperature treatment step of subjecting the water-absorbent
resin particles to heat treatment at 100.degree. C. or higher, a
step of performing ultraviolet irradiation, a step of using a
bactericide, and a step of using a gas of an alkylating agent such
as ethylene oxide or formaldehyde in the presence of ozone.
Examples of the bactericide include an aqueous solution in which
ozone is dissolved and hypochlorous acid (salt). The step of
subjecting water-absorbent resin particles to sterilization
treatment is preferably a step of using a bactericide from the
viewpoint of safety, and more preferably a step of using
hypochlorous acid (salt).
[0122] The oxidation-treated decomposition product (B) may be
further subjected to reduction of the water content in the
decomposition product with a dehydrating agent.
[0123] The dehydrating agent is not particularly limited, and known
dehydrating agents, for example, calcium salts or magnesium salts,
such as calcium chloride, magnesium chloride and calcium acetate,
strong acids such as hydrochloric acid, sulfuric acid and nitric
acid, and hydroxy acids such as citric acid, isocitric acid and
malic acid can be applied.
[0124] The amount of the dehydrating agent added is 0.1 to 20 based
on the solid content (% by weight) of the decomposition product
(B). It is preferably 0.2 to 10, and more preferably 0.5 to 10.
Within these ranges, dehydration efficiently proceeds from the
inside of the decomposition product (B), so that handling is easy
at the time of recycling. When the amount of the dehydrating agent
added is 0.1 or less, dehydration does not proceed efficiently, and
it is undesirable that the amount of the dehydrating agent added is
20 or more because, if so, an excessively remaining dehydrating
agent inhibits polymerization or crosslinking.
[0125] In the method for producing water-absorbent resin particles
of the present invention, as described above, the decomposition
product (B) of water-absorbent resin particles is added in any step
before the surface-crosslinking reaction step (Vb) of reacting with
the surface-crosslinking agent (d), that is, at least in one of the
steps of, for example, the polymerization step (I), the gel
chopping step (II), the drying step (III), the pulverization
classification step (IV), and the step (Va) of mixing resin
particles with the surface-crosslinking agent (d). Addition at the
time of these steps makes it possible to efficiently produce a
water-absorbent resin having a small deterioration in absorption
characteristics and various superior characteristics. In
particular, from the viewpoint of improvement in water absorption
performance and stabilization of quality, the decomposition product
(B) of water-absorbent resin particles is preferably added in the
polymerization step (I) and/or the gel chopping step (II), and it
is more preferably added in the gel chopping step (II).
[0126] The method of adding the decomposition product (B) of
water-absorbent resin particles is not particularly limited as long
as it is a method capable of adding the decomposition product in
any step before the surface-crosslinking reaction step (Vb) of
reacting with the surface-crosslinking agent (d), and examples
thereof include a method in which a treatment liquid resulting from
decomposition treatment in water is directly added and a method in
which the decomposition product having been subjected to
decomposition treatment in water and then dried is added.
[0127] When the decomposition product (B) of water-absorbent resin
particles is added in the polymerization step (I), the
decomposition product (B) may be uniformly dissolved in an aqueous
acrylic acid solution or may be dispersed in an aqueous acrylic
acid solution. The form of the (B) is preferably a gel form or a
liquid form. The decomposition product (B) to be added may be
subjected to an operation of removing metal ions remaining in the
decomposition product (B) in advance using an ion exchange resin or
complex ions as necessary, and the content of the remaining metal
can be measured by ICP emission spectrometry or the like.
[0128] When the decomposition product (B) of water-absorbent resin
particles is added in the polymerization step (I), the weight ratio
of the decomposition product (B) of water-absorbent resin particles
to the water-soluble vinyl monomer (a1) is 5/95 to 95/5, more
preferably 5/95 to 80/20, and still more preferably 10/90 to 70/30.
Within these ranges, the recycling efficiency and the absorption
performance of the water-absorbent resin particles after recycling
are favorably obtained.
[0129] When the decomposition product (B) of water-absorbent resin
particles is added in the gel chopping step (II), the form of the
(B) may be a powdery form, a gel form, or a liquid form. The number
of times of chopping may be increased or decreased as necessary, or
the decomposition product (B) may be added in two or more portions,
and from the viewpoint of the absorption performance of the
recycled water-absorbent resin particles comprising the (B), it is
preferable that the hydrous gel comprising the crosslinked polymer
(A) and the (B) are kneaded while being chopped.
[0130] The apparatus capable of drying the mixture of the hydrous
gel comprising the crosslinked polymer (A) and the decomposition
product (B) is not particularly limited, and a concurrent band
dryer (tunnel dryer), a ventilated band dryer, spouting stream
(nozzle jet) dryer, a ventilated vertical dryer, a box type hot air
dryer, etc. are suitably used. A plurality of these may be used in
combination. The heat sources (steam, heat medium, etc.) of these
dryers are not particularly limited.
[0131] The range of the mixing weight ratio of the hydrous gel
comprising the crosslinked polymer (A) to the decomposition product
(B) is preferably 99/1 to 10/90, and more preferably 97/3 to 20/80.
Within these ranges, the dryability is not impaired and the
required water absorption performance can be satisfied.
[0132] When the decomposition product (B) of water-absorbent resin
particles is added in the gel chopping step (II), it is more
preferable to add the decomposition product (B) before and/or
simultaneously with the gel chopping. When the decomposition
product (B) of water-absorbent resin particles is added in the
polymerization step, the polymerization behavior may be adversely
affected due to the influence of impurities contained in the
decomposition product (B). On the other hand, when the
decomposition product (B) is added before and/or simultaneously
with the gel chopping, the absorption characteristics are less
deteriorated and stable absorption characteristics can be obtained.
Here, the gel chopping step refers to a series of steps in which
the hydrous gel is charged into a pulverizing apparatus and the
pulverizing apparatus is operated to obtain chopped gel, the
addition before the gel chopping refers to charging the
decomposition product (B) into the pulverizing apparatus in advance
before the hydrous gel is charged into the pulverizing apparatus,
and the addition simultaneously with the gel chopping refers to
simultaneously adding the hydrous gel and the decomposition product
(B) to the pulverizing apparatus.
[0133] From the viewpoint of recycling efficiency and absorption
performance of recycled water-absorbent resin particles, the
content (% by weight) of the decomposition product (B) of
water-absorbent resin particles is preferably 1 to 50, more
preferably 1 to 40, and particularly preferably 1 to 30, based on
the total weight of the monomer composition.
[0134] The weight average particle diameter (.mu.m) of the
water-absorbent resin particles obtained by the production method
of the present invention is preferably 150 to 600, and more
preferably 200 to 500 from the viewpoint of absorption performance
and handleability.
[0135] The weight average particle diameter is measured by the
method disclosed in Perry's Chemical Engineers' Handbook, Sixth
Edition (McGraw-Hill Book Company, 1984, page 21) by using a RO-TAP
screen shaker and standard screens (JIS Z8801-1:2006).
Specifically, JIS standard sieves are combined, for example, in the
order of 1000 .mu.m, 850 .mu.m, 710 .mu.m, 500 .mu.m, 425 .mu.m,
355 .mu.m, 250 .mu.m, 150 .mu.m, 125 .mu.m, 75 .mu.m, 45 .mu.m, and
a bottom tray when viewed from the top. About 50 g of particles to
be measured are put on the top screen and then shaken for five
minutes by a RO-TAP screen shaker. Then, the particles to be
measured received on the respective screens and the bottom tray are
weighed and the weight fractions of the particles on the respective
screens are calculated with the total weight of the particles
considered to be 100% by weight. The calculated values are plotted
on a logarithmic probability sheet {taking the size of openings of
a screen (particle diameter) as abscissa and the weight fraction as
ordinate} and then a line connecting the respective points is
drawn. Subsequently, a particle diameter that corresponds to a
weight fraction of 50% by weight is determined and this is defined
as a weight average particle diameter.
[0136] The shape of the water-absorbent resin particles is not
particularly limited and may be an irregularly pulverized form, a
scaly form, a pearl-like form, a rice grain form, etc. Among these,
an irregularly pulverized form is preferred because good entangling
with a fibrous material in an application such as disposable diaper
is ensured and the fear of falling off from the fibrous material is
eliminated.
[0137] The apparent density (g/ml) of the water-absorbent resin
particles is preferably 0.40 to 0.80, more preferably 0.50 to 0.75,
and particularly preferably 0.55 to 0.70, from the viewpoint of the
absorption performance of sanitary goods. The apparent density is
measured at 25.degree. C. in accordance with JIS K 7365: 1999.
[0138] The water retention capacity (g/g) for physiological saline
of water-absorbent resin particles is 20 to 60, and preferably 25
to 55, from the viewpoint of the absorption performance of sanitary
goods. The water retention capacity for physiological saline is
measured by the following method.
<Water Retention Capacity for Physiological Saline>
[0139] 1.00 g of a measurement sample is put into a tea bag (20 cm
long, 10 cm wide) made of nylon net with a size of openings of 63
.mu.m (JIS Z8801-1:2006) and then is immersed in 1,000 ml of
physiological saline (salt concentration: 0.9% by weight) for 1
hour without stirring, followed by draining off water by hanging
the sample for 15 minutes. Then, the sample in the tea bag is put
in a centrifuge and centrifugally dewatered at 150 G for 90
seconds, thereby removing excess physiological saline.
Subsequently, the weight (h1) of the sample including the tea bag
is measured and then a water retention capacity is calculated from
the following formula.
Water retention capacity (g/g) for physiological
saline=(h1)-(h2)
[0140] The temperature of the physiological saline used and that of
the measurement atmosphere are 25.degree. C..+-.2.degree. C. The
weight of the tea bag after centrifugal dehydration is measured in
the same manner as described above except that the measurement
sample is not used and the measured weight is denoted by (h2).
[0141] The amount of absorption under load (g/g) of the
water-absorbent resin particles is preferably 20 or more. It is
undesirable that the amount of absorption under load is less than
20 because, if so, leakage is likely to occur during repeated use.
The upper limit is preferably 27 or less from the viewpoint of
performance balance with other physical properties and productivity
although the upper limit is preferably as high as possible and not
particularly limited. The amount of absorption under load can be
appropriately adjusted by the types and the amounts of the
crosslinking agent (b) and the surface-crosslinking agent (d).
Therefore, for example, when it is necessary to increase the amount
of absorption under load, it can be easily realized by increasing
the used amount of the crosslinking agent (b) and the
surface-crosslinking agent (d).
<Method for Measuring the Amount of Absorption Under
Load>
[0142] Into a cylindrical plastic tube (inner diameter: 25 mm,
height: 34 mm) with a nylon net having a size of openings of 63
.mu.m (JIS Z8801-1:2006) attached to the bottom of the tube, there
is weighed 0.16 g of a measurement sample screened into a range of
250 to 500 .mu.m using standard screens, and then the cylindrical
plastic tube is made to stand vertically and the measurement sample
is leveled to have an almost uniform thickness on the nylon net and
then a weight (weight: 310.6 g, outer diameter: 24.5 mm) is put on
the measurement sample. The weight (M1) of the cylindrical plastic
tube as a whole is measured, and then the cylindrical plastic tube
containing the measurement sample and the weight is made to stand
in a petri dish (diameter: 12 cm) containing 60 ml of physiological
saline (salt concentration: 0.9%) and is immersed with the nylon
net side facing down and is left standing for 60 minutes. After a
lapse of 60 minutes, the cylindrical plastic tube was pulled up
from the petri dish and then was inclined to concentrate the water
attaching to the bottom of the tube to drip in the form of water
drops, thereby removing excess water. Then, the weight (M2) of the
cylindrical plastic tube containing the measurement sample and the
weight as a whole was measured and then the amount of absorption
under load is determined from the following formula. The
temperature of the physiological saline used and that of the
measurement atmosphere are 25.degree. C..+-.2.degree. C.
The amount (g/g) of absorption under load={(M2)-(M1)}/0.16
EXAMPLES
[0143] The present invention is further described below by means of
Examples and Comparative Examples, but the present invention is not
limited thereto. Unless otherwise stated, "part(s)" means "part(s)
by weight" and "%" means "% by weight."
Production Example 1
[0144] 157 parts (2.18 parts by mol) of acrylic acid, 0.6305 parts
(0.0024 parts by mol) of a crosslinking agent (b) {pentaerythritol
triallyl ether}, and 344.65 parts of deionized water were kept at
3.degree. C. while being stirred and mixed. After adjusting the
dissolved oxygen amount to 1 ppm or less by introducing nitrogen
into this mixture, 0.63 parts of a 1% aqueous hydrogen peroxide
solution, 1.1774 parts of a 2% aqueous ascorbic acid solution, and
2.355 parts of a 2% aqueous
2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide] solution were
added and mixed, so that polymerization was initiated. After the
temperature of the mixture reached 90.degree. C., polymerization
was performed at 90.+-.2.degree. C. for about 5 hours, thereby
affording hydrous gel (1). Then, while chopping 502.27 parts of the
hydrous gel (1) with a mincing machine, 128.42 parts of a 48.5%
aqueous sodium hydroxide solution was added and mixed, thereby
affording hydrous gel particles. Further, the hydrous gel particles
were dried in a through-air band dryer (150.degree. C., wind speed:
2 m/sec), affording a dried material. The dried material was
pulverized with a juicing blender and then adjusted it into a
particle diameter range of from 710 to 150 .mu.m, thereby affording
dried material particles (1). While stirring 100 parts of the dried
material particles (1) at a high speed, 0.5 parts of sodium
aluminum sulfate dodecahydrate (sodium alum) as a multivalent metal
salt (e) and 5.00 parts of a 2% solution of ethylene glycol
diglycidyl ether in a water/methanol mixed solvent (weight ratio of
water/methanol=70/30) were added thereto by spraying and mixed, and
the resulting mixture was allowed to stand at 150.degree. C. for 30
minutes for surface-crosslinking, thereby affording water-absorbent
resin particles (P-1). The weight average particle diameter of
(P-1) was 400 .mu.m.
Production Example 2
[0145] 30.0 g of the water-absorbent resin particles (P-1) obtained
in Production Example 1 were put in a PE wide-mouth bottle 10 L
(manufactured by SANPLATEC Corporation), and 1500 ml of
ion-exchanged water in which 5.0 g of ascorbic acid (manufactured
by FUJIFILM Wako Pure Chemical Corporation) and 0.7 g of iron(II)
sulfate heptahydrate (manufactured by FUJIFILM Wako Pure Chemical
Corporation) were dissolved was further added and allowed to stand
for 20 minutes, thereby swelling the water-absorbent resin
particles. Then, the water-absorbent resin particles in a swollen
state were dispersed and homogenized at 1500 rpm for 10 minutes
using a homogenizer (product name: EXCEL AUTO HOMOGENIZER,
manufactured by NIHONSEIKI Kaisha Ltd.). While the swollen
water-absorbent resin particles attached to the homogenizer were
collected, the homogenizer was removed, and then the upper part of
the PE wide-mouth bottle was covered with a PVCA film (Saran film),
and the PVCA film was fixed with a rubber band so as not to come
off, thereby forming a sealed state. Subsequently, this was allowed
to stand for 10 hours in a thermostat (model: IG401, manufactured
by Yamato Scientific Co., Ltd.) set at 70.degree. C. to perform
decomposition treatment of the water-absorbent resin particles.
After the decomposition treatment, the obtained treatment liquid
was freeze-dried, affording a decomposition product (B-1) of the
water-absorbent resin particles. The content of water solubles of
(B-1) was 82% by weight.
Production Example 3
[0146] 30.0 g of the water-absorbent resin particles (P-1) obtained
in Production Example 1 were put in a PE wide-mouth bottle 10 L
(manufactured by SANPLATEC Corporation), and 1500 ml of
ion-exchanged water in which 5.0 g of ascorbic acid (manufactured
by FUJIFILM Wako Pure Chemical Corporation) and 0.7 g of iron(II)
sulfate heptahydrate (manufactured by FUJIFILM Wako Pure Chemical
Corporation) were dissolved was further added and 30.0 g of a 1.0
wt % aqueous sodium hypochlorite solution was further added as a
bactericide, and allowed to stand for 20 minutes, thereby swelling
the water-absorbent resin particles. Then, the upper part of the PE
wide-mouth bottle was covered with a PVCA film (Saran film), and
the PVCA film was fixed with a rubber band so as not to come off,
thereby forming a sealed state. Subsequently, this was allowed to
stand for 10 hours in a thermostat (model: IG401, manufactured by
Yamato Scientific Co., Ltd.) set at 70.degree. C. to perform
decomposition treatment of the water-absorbent resin particles.
After the decomposition treatment, the obtained treatment liquid
was freeze-dried, affording a decomposition product (B-2) of
water-absorbent resin particles. The content of water solubles of
(B-2) was 79% by weight. This content of water solubles is a value
obtained by subtracting the quantified amount of the acid component
from a measured value of the content of water solubles.
Production Example 4
[0147] 5.0 g of the water-absorbent resin particles (P-1) obtained
in Production Example 1 were separated into 1.000-gram portions
each in a tea bag (20 cm long, 10 cm wide) formed of nylon net
having a mesh size of 63 .mu.m (JIS Z 8801-1: 2006), and each of
them was transferred to a 1 L conical beaker (model:
010050-100061A: manufactured by AS ONE Corporation) containing 500
ml of 0.9 wt % physiological saline and 1.0 g of 1.0 wt % aqueous
sodium hypochlorite solution, and allowed to stand for 60 minutes,
thereby swelling the water-absorbent resin particles. Then, the
whole tea bag was transferred to a 1 L conical beaker (the same as
above) containing 500 ml of a 1.0 wt % aqueous calcium chloride
solution and allowed to stand for 60 minutes, then the whole tea
bag was placed in a centrifugal separator, and centrifugally
dehydrated at 150 G for 90 seconds to remove excessive water,
thereby affording a decomposition product (B-3) of the
water-absorbent resin particles. The content of water solubles of
(B-3) was 32% by weight. This content of water solubles is a value
obtained by subtracting the quantified amount of the acid component
from a measured value of the content of water solubles. The average
water retention capacity at this time was 3 (g/g).
Production Example 5
[0148] 5.0 g of the water-absorbent resin particles (P-1) obtained
in Production Example 1 were separated into 1.000-gram portions
each in a tea bag (20 cm long, 10 cm wide) formed of nylon net
having a mesh size of 63 .mu.m (JIS Z 8801-1: 2006), and each of
them was transferred to a 1 L conical beaker (model:
010050-100061A: manufactured by AS ONE Corporation) containing 500
ml of 0.9 wt % physiological saline and 1.0 g of 1.0 wt % aqueous
sodium hypochlorite solution, and allowed to stand for 60 minutes,
thereby swelling the water-absorbent resin particles. Then, the
swollen water-absorbent resin particles were taken out from the tea
bag, spread on a stainless steel tray (20 cm.times.50 cm) so as to
be as uniform as possible, and allowed to stand in a dryer set at
150.degree. C. for 1 hour to dry, affording a decomposition product
(B-4) of the water-absorbent resin particles. The content of water
solubles of (B-4) was 33% by weight. This content of water solubles
is a value obtained by subtracting the quantified amount of the
acid component from a measured value of the content of water
solubles.
Production Example 6
[0149] 5.0 g of the water-absorbent resin particles (P-1) obtained
in Production Example 1 were separated into 1.000-gram portions
each in a tea bag (20 cm long, 10 cm wide) formed of nylon net
having a mesh size of 63 .mu.m (JIS Z 8801-1: 2006), and each of
them was transferred to a 1 L conical beaker (model:
010050-100061A: manufactured by AS ONE Corporation) containing 500
ml of 0.9 wt % physiological saline and 1.0 g of 1.0 wt % aqueous
sodium hypochlorite solution, and allowed to stand for 60 minutes,
thereby swelling the water-absorbent resin particles. Then, the
whole tea bag was transferred to a 1 L conical beaker (the same as
above) containing 500 ml of a 1.0 wt % aqueous calcium chloride
solution and allowed to stand for 60 minutes, then the whole tea
bag was placed in a centrifugal separator, and centrifugally
dehydrated at 150 G for 90 seconds to remove excessive water. The
resulting product was allowed to stand in a dryer set at
150.degree. C. for 1 hour to dry, affording a decomposition product
(B-5) of the water-absorbent resin particles. The content of water
solubles of (B-5) was 33% by weight. This content of water solubles
is a value obtained by subtracting the quantified amount of the
acid component from a measured value of the content of water
solubles. The average water retention capacity at this time was 3
(g/g).
Production Example 7
[0150] 5.0 g of the water-absorbent resin particles (P-1) obtained
in Production Example 1 were separated into 1.000-gram portions
each in a tea bag (20 cm long, 10 cm wide) formed of nylon net
having a mesh size of 63 .mu.m (JIS Z 8801-1: 2006), and each of
them was transferred to a 1 L conical beaker (model:
010050-100061A: manufactured by AS ONE Corporation) containing 500
ml of 0.9 wt % physiological saline and 1.0 g of 1.0 wt % aqueous
sodium hypochlorite solution, and allowed to stand for 60 minutes,
thereby swelling the water-absorbent resin particles. Subsequently,
a 1N aqueous sodium hydroxide solution was added such that the pH
of the aqueous solution was 7.0. The swollen water-absorbent resin
particles were taken out from the tea bag, spread on a stainless
steel tray (20 cm.times.50 cm) so as to be as uniform as possible,
and allowed to stand in a dryer set at 150.degree. C. for 1 hour to
dry, affording a decomposition product (B-6) of the water-absorbent
resin particles. The content of water solubles of (B-6) was 35% by
weight.
Production Example 8
[0151] 157 parts (2.18 parts by mol) of a water-soluble vinyl
monomer (a1) {acrylic acid}, 310 parts (0.0031 parts by mol) of an
internal crosslinking agent (b-1) {ALKOX CP-A1H; number of
crosslinkable functional groups: 23, number average molecular
weight: 100,000; manufactured by Meisei Chemical Works, Ltd.}, and
344.65 parts of deionized water were kept at 3.degree. C. while
being stirred and mixed. After adjusting the dissolved oxygen
amount to 1 ppm or less by introducing nitrogen into this mixture,
0.63 parts of a 1% aqueous hydrogen peroxide solution, 1.1774 parts
of a 2% aqueous ascorbic acid solution, and 2.355 parts of a 2%
aqueous 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide]
solution were added and mixed, so that polymerization was
initiated. After the temperature of the mixture reached 90.degree.
C., polymerization was performed at 90.+-.2.degree. C. for about 5
hours, thereby affording hydrous gel (2). Then, while chopping
502.27 parts of the hydrous gel (2) with a mincing machine, 128.42
parts of a 48.5% aqueous sodium hydroxide solution was added and
mixed, thereby affording hydrous gel particles. Further, the
hydrous gel particles were dried in a through-air band dryer
(150.degree. C., wind speed: 2 m/sec), affording a dried material.
The dried material was pulverized with a juicing blender and then
adjusted it into a particle diameter range of from 710 to 150
.mu.m, thereby affording dried material particles (2). While
stirring 100 parts of the dried material particles at a high speed,
0.5 parts of sodium aluminum sulfate dodecahydrate (sodium alum) as
a multivalent metal salt (e) and 5.00 parts of a 2% solution of
ethylene glycol diglycidyl ether in a water/methanol mixed solvent
(weight ratio of water/methanol=70/30) were added thereto by
spraying and mixed, and the resulting mixture was allowed to stand
at 150.degree. C. for 30 minutes for surface-crosslinking, thereby
affording water-absorbent resin particles (P-2). The weight average
particle diameter of (P-2) was 400 .mu.m.
Production Example 9
[0152] 5.0 g of the water-absorbent resin particles (P-2) obtained
in Production Example 8 were transferred to a 1 L conical beaker
(model: 010050-100061A, manufactured by AS ONE Corporation), and
150 g of 0.9 wt % physiological saline and 1 ml of a 1N aqueous
sodium hydroxide solution were further added to swell the
water-absorbent resin particles. Subsequently, this was allowed to
stand for 20 hours in a thermostat (model: IG401, manufactured by
Yamato Scientific Co., Ltd.) set at 80.degree. C. to perform
decomposition treatment of the water-absorbent resin particles,
thereby affording a decomposition product (B-7) of the
water-absorbent resin particles. The content of water solubles of
(B-7) was 79% by weight.
Production Example 10
[0153] For a commercially available diaper (Product name "Attento
thin dry touch pants" M to L sizes, manufactured by Daio Paper
Corporation) after urination, the entire diaper was washed using 2
L of ion-exchanged water and then chopped with scissors, and the
swollen water-absorbent resin particles contained in the diaper
were taken out in a state of being mixed with pulp. 100 g of the
mixture of swollen water-absorbent resin particles and pulp was
transferred to a 3LPE wide-angle bottle (model: 2088, manufactured
by SANPLATEC Corporation) containing 1500 ml of 0.9 wt %
physiological saline and 3.0 g of 1.0 wt % aqueous sodium
hypochlorite solution, and allowed to stand for 60 minutes. The
mixture was immersed in 2 L of a 1.0 wt % calcium chloride solution
for 60 minutes and then taken out, and then the water-absorbent
resin particles were separated from the pulp using the difference
in specific gravity, thereby affording a decomposition product
(B-8). The content of water solubles of (B-8) was 31% by weight.
This content of water solubles is a value obtained by subtracting
the quantified amount of the acid component from a measured value
of the content of water solubles. The water retention capacity of
the water-absorbent resin particles at this time was 3 g/g.
Example 1
[0154] Dried material particles were obtained by performing the
same operations as those of Production Example 1 except, in
Production Example 1, changing the quantity of acrylic acid form
157 parts to 141.3 parts and the quantity of a 48.5% aqueous sodium
hydroxide solution from 128.42 parts to 115.58 parts, and
dissolving 14.1 parts of the decomposition product (B-1) in 344.65
parts of deionized water in advance and using the resulting aqueous
solution for polymerization. Subsequently, water-absorbent resin
particles (P-3) were obtained by performing the same operations as
those of Production Example 1 except, in the surface-crosslinking
step, changing the quantity of the 2% solution of ethylene glycol
diglycidyl ether in a water/methanol mixed solvent (weight ratio of
water/methanol=70/30) from 5.00 parts to 6.50 parts.
Example 2
[0155] 452.04 parts of the hydrous gel (1) obtained in Production
Example 1 and 14.1 parts of the decomposition product (B-1) were
mixed, and then while the mixture was chopped with a mincing
machine, 115.58 parts of a 48.5% aqueous sodium hydroxide solution
was added and mixed, thereby affording hydrous gel particles.
Further, the hydrous gel particles were dried in a through-air band
dryer (150.degree. C., wind speed: 2 m/sec), affording a dried
material. Subsequently, water-absorbent resin particles (P-4) were
obtained by performing the same operations as those of Production
Example 1 except, in the surface-crosslinking step, changing the
quantity of the 2% solution of ethylene glycol diglycidyl ether in
a water/methanol mixed solvent (weight ratio of
water/methanol=70/30) from 5.00 parts to 7.00 parts.
Example 3
[0156] While 452.04 parts of the hydrous gel (1) obtained in
Production Example 1 were chopped with a mincing machine, 115.58
parts of a 48.5% aqueous sodium hydroxide solution was added and
mixed, affording hydrous gel particles. The obtained hydrous gel
particles were mixed with 14.1 parts of the decomposition product
(B-1) and then dried in a through-air band dryer (150.degree. C.,
wind speed: 2 m/sec), affording a dried material. Subsequently,
water-absorbent resin particles (P-5) were obtained by performing
the same operations as those of Production Example 1 except, in the
surface-crosslinking step, changing the quantity of the 2% solution
of ethylene glycol diglycidyl ether in a water/methanol mixed
solvent (weight ratio of water/methanol=70/30) from 5.00 parts to
7.00 parts.
Example 4
[0157] Water-absorbent resin particles (P-6) were obtained by
performing the same operations as those of Example 1 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-2).
Example 5
[0158] Water-absorbent resin particles (P-7) were obtained by
performing the same operations as those of Example 2 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-2).
Example 6
[0159] Water-absorbent resin particles (P-8) were obtained by
performing the same operations as those of Example 2 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-3).
Example 7
[0160] 100 parts of the dried material obtained in Production
Example 1 was pulverized with a juicing blender, and then 10 parts
of the decomposition product (B-3) was added at the time of
adjusting the particle diameter to a particle diameter range of
from 710 to 150 .mu.m, thereby affording a dried particles (2).
[0161] While stirring 100 parts of the dried particles (2) at a
high speed, 0.5 parts of sodium aluminum sulfate dodecahydrate
(sodium alum) as a multivalent metal salt (e) and 5.00 parts of a
2% solution of ethylene glycol diglycidyl ether in a water/methanol
mixed solvent (weight ratio of water/methanol=70/30) were added
thereto by spraying and mixed, and the resulting mixture was
allowed to stand at 150.degree. C. for 30 minutes for
surface-crosslinking, thereby affording water-absorbent resin
particles (P-9). The weight average particle diameter of (P-9) was
400 .mu.m.
Example 8
[0162] Water-absorbent resin particles (P-10) were obtained by
performing the same operations as those of Example 1 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-4).
Example 9
[0163] Water-absorbent resin particles (P-11) were obtained by
performing the same operations as those of Example 2 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-4).
Example 10
[0164] Water-absorbent resin particles (P-12) were obtained by
performing the same operations as those of Example 2 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-5).
Example 11
[0165] Water-absorbent resin particles (P-13) were obtained by
performing the same operations as those of Example 2 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-6).
Example 12
[0166] 100 parts of the dried material obtained in Production
Example 1 was pulverized with a juicing blender, and then 10 parts
of the decomposition product (B-3) was added at the time of
adjusting the particle diameter to a particle diameter range of
from 710 to 150 .mu.m, thereby affording a dried particles (3).
[0167] While stirring 100 parts of the dried particles (3) at a
high speed, 0.5 parts of sodium aluminum sulfate dodecahydrate
(sodium alum) as a multivalent metal salt (e) and 5.00 parts of a
2% solution of ethylene glycol diglycidyl ether in a water/methanol
mixed solvent (weight ratio of water/methanol=70/30) were added
thereto by spraying and mixed, and the resulting mixture was
allowed to stand at 150.degree. C. for 30 minutes for
surface-crosslinking, thereby affording water-absorbent resin
particles (P-14). The weight average particle diameter of (P-14)
was 400 .mu.m.
Example 13
[0168] Water-absorbent resin particles (P-15) were obtained by
performing the same operations as those of Example 1 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-7).
Example 14
[0169] Water-absorbent resin particles (P-16) were obtained by
performing the same operations as those of Example 2 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-7).
Example 15
[0170] Water-absorbent resin particles (P-17) were obtained by
performing the same operations as those of Example 2 except
changing 14.1 parts of the decomposition product (B-1) to 14.1
parts of the decomposition product (B-8).
Example 16
[0171] To 100 parts of the dried material particles (1) obtained in
Production Example 1 was added 10 parts of the decomposition
product (B-1) and the mixture was kneaded on a stainless steel tray
so as to be uniform. Then, 0.5 parts of sodium aluminum sulfate
dodecahydrate (sodium alum) as a multivalent metal salt (e) and
5.00 parts of a 2% solution of ethylene glycol diglycidyl ether in
a water/methanol mixed solvent (weight ratio of
water/methanol=70/30) were added thereto by spraying and mixed, and
the resulting mixture was allowed to stand at 150.degree. C. for 30
minutes for surface-crosslinking, thereby affording water-absorbent
resin particles (P-18).
Comparative Example 1
[0172] To 90 parts of the water-absorbent resin particles (P-1)
obtained in Production Example 1 was added 10 parts of the
decomposition product (B-1), and the mixture was kneaded on a
stainless steel tray so as to be uniform, thereby affording
water-absorbent resin particles (H-1).
Comparative Example 2
[0173] To 95 parts of the water-absorbent resin particles (P-1)
obtained in Production Example 1 was added 5 parts of the
decomposition product (B-1), and the mixture was kneaded on a
stainless steel tray so as to be uniform, thereby affording
water-absorbent resin particles (H-2).
Comparative Example 3
[0174] To 90 parts of the water-absorbent resin particles (P-1)
obtained in Production Example 1 was added 10 parts of the
decomposition product (B-4), and the mixture was kneaded on a
stainless steel tray so as to be uniform, thereby affording
water-absorbent resin particles (H-3).
Comparative Example 4
[0175] To 90 parts of the water-absorbent resin particles (P-1)
obtained in Production Example 1 was added 10 parts of the
decomposition product (B-6), and the mixture was kneaded on a
stainless steel tray so as to be uniform, thereby affording
water-absorbent resin particles (H-4).
[0176] The evaluation results of the water retention capacity and
the amount of absorption under load of the water-absorbent resin
particles obtained in Production Examples, Examples, and
Comparative Examples are shown in Table 1.
TABLE-US-00001 TABLE 1 Amount of Water-absorbent Water retention
absorption under resin particles capacity (g/g) load (g/g)
Production P-1 40.3 26.0 Example 1 Production P-2 40.0 24.9 Example
2 Example 1 P-3 39.9 25.5 Example 2 P-4 40.2 25.0 Example 3 P-5
39.5 25.5 Example 4 P-6 39.7 25.6 Example 5 P-7 40.2 26.1 Example 6
P-8 40.3 24.9 Example 7 P-9 39.2 24.7 Example 8 P-10 39.5 25.5
Example 9 P-11 39.7 25.6 Example 10 P-12 39.8 24.7 Example 11 P-13
40.3 25.4 Example 12 P-14 40.0 25.4 Example 13 P-15 40.5 24.6
Example 14 P-16 39.8 26.3 Example 15 P-17 40.3 25.0 Comparative H-1
39.9 16.2 Example 1 Comparative H-2 40.2 17.2 Example 2 Comparative
H-3 41.4 14.9 Example 3 Comparative H-4 40.2 14.4 Example 4
Comparative H-5 39.5 18.0 Example 5
[0177] As is apparent from the results shown in Table 1, the
water-absorbent resin particles obtained by the production method
of the present invention using the decomposition products of the
water-absorbent resin particles shown in Examples are improved in
water retention capacity and amount of absorption under load as
compared with the water-absorbent resin particles obtained by
adding the decomposition products to steps after
surface-crosslinking shown in Comparative Examples. In addition, it
is found that deterioration of absorption characteristics is
suppressed also from the comparison of Production Example 1 in
which a decomposition product of water-absorbent resin particles is
not added with Examples of the present application.
[0178] From the present results, it can be said that the present
invention is an efficient production method from the viewpoint of
resource saving and reduction in environmental load because the
obtained water-absorbent resin particles exhibit good absorption
characteristics even when a decomposition product of
water-absorbent resin particles, which is a waste of
water-absorbent resin particles, is added in a step before the
surface-crosslinking step and is recycled as a raw material.
INDUSTRIAL APPLICABILITY
[0179] The method for producing water-absorbent resin particles of
the present invention can be suitably used for recycling waste of
water-absorbent resin particles discharged when sanitary goods such
as disposable diapers (disposable diapers for children, disposable
diapers for adults, etc.), napkins (sanitary napkins, etc.), paper
towels, pads (incontinence pads, surgical underpads, etc.), and pet
sheets (pet urine absorbent sheets) are recycled.
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